The present invention relates to a method for estimating the temperature of a catalytic converter or possible brief increases in the temperature of the catalytic converter, methods for controlling the mixture supply of an internal combustion engine connected upstream from the catalytic converter, which use the aforementioned methods, and an engine control apparatus for carrying out a method of this type.
An exhaust gas catalytic converter is connected downstream from internal combustion engines of modern motor vehicles, in particular petrol engines, and is used to reduce pollutants contained in the exhaust gas, such as, for example, nitrogen oxides, carbon monoxides and non-combusted hydrocarbons. So that these catalytic converters operate efficiently, they have to be operated at a high temperature and with a stoichiometric air ratio (lambda=1). However, severe aging effects occur in the currently used catalytic converters at temperatures above 950° C. In order to ensure a long service life for the catalytic converters, temperatures which are too high therefore have to be avoided. In other words, the temperature of the catalytic converter must be known and ways have to be known for influencing it.
For cost reasons, the catalytic converter temperature in mass-produced vehicles is not measured, but calculated in an engine control apparatus. A method for this is described in U.S. Pat. No. 5,414,994.
It is known to control the temperature of a catalytic converter of this type by means of the air ratio of the mixture supplied to the engine. By using a rich mixture, the supply of oxygen to the catalytic converter is restricted and as a result of this, less heat is released in the catalytic converter owing to the catalytic oxidation of the remaining hydrocarbons and carbon monoxides contained in the exhaust gas. In addition, the enriching of the mixture reduces the temperature of the exhaust gas supplied to the catalytic converter, as the latter is cooled by the evaporation of the fuel supplied to excess.
Owing to the drawbacks connected therewith regarding fuel consumption and emissions, the aim is to avoid such enriching of the mixture as far as possible.
When the internal combustion engine is operated for a time with a rich mixture, non-combusted hydrocarbons and carbon monoxide collect in the catalytic converter. As soon as only the smallest quantities of oxygen are present in the exhaust gas flow supplied to the catalytic converter, i.e. on transition to a lean mixture, these hydrocarbons are abruptly converted in the catalytic converter and this leads to a corresponding increase in temperature.
Experimentally, during transition from full load operation with a rich mixture (λ=0.9) after deceleration fuel cut-off, temperature increases of 5 to 30 K were measured within less than 10 ms.
Conversely, the catalytic converter, after deceleration fuel cut-off, is saturated with oxygen within a very short time; it is in a position to store several g thereof. In a subsequent loading with a rich mixture, for example by full load enriching, a still more severe reaction occurs in the catalytic converter.
The calculation method known from U.S. Pat. No. 5,414,994 does not take into account this transient heating. During dynamic driving operation with transitions between a rich and lean mixture, substantial differences may therefore occur between the actual catalytic converter temperature and that calculated with the aid of this conventional calculation model. This fact is conventionally taken into account in that the catalytic converter temperature is controlled to a maximum desired temperature, which is below the critical temperature of, for example, 950° C. with a safety margin of 30 to 50 K. At constant driving at a high load, this restriction of the catalytic temperature leads to a fuel consumption which is higher by about 2 to 5% in comparison to an engine, which is operated with a catalytic converter operating at the critical temperature.